The present invention relates to functionalized diene polymers having improved properties of use in unvulcanized state and hysteresis properties in vulcanized state in elastomeric compositions comprising silica as filler, to their use in tires, and to a new process of preparing the said functionalized polymers.
Since savings in fuel and the necessity of protecting the environment have become a priority, it is desirable to produce polymers having good mechanical properties and a hysteresis which is as slight as possible in order to be able to use them in the form of rubber compositions which can be employed for the manufacture of various semi-finished products entering into the constitution of tires, such as, for instance, under-layers, bonding rubbers between rubbers of different kinds or calendering rubbers for metal or textile reinforcements, sidewall rubbers, or treads, and to obtain tires having improved properties, in particular reduced resistance to rolling.
In order to achieve this, numerous solutions have been proposed, which consist, in particular, of modifying, inter alia, the nature of the diene polymers and copolymers for polymerization by means of coupling or starring agents or functionalizing agents. The great majority of these solutions are concentrated essentially on the use of polymers modified with carbon black as reinforcing filler in order to obtain good interaction between the modified polymer and the carbon black, since the use of white reinforcing fillers, and in particular of silica, has proven unsuitable due to the low level of certain properties of such compositions and, therefore, of certain properties of tires using these compositions. Furthermore, the use of silica raises difficulties in working due to silica/silica interactions which tend in the raw state to cause an agglomeration of the silica particles before and even after mixing and thus to increase the consistency of the rubber compositions, and in any event to make the use of them more difficult than the use of carbon black. By way of illustrative examples of this prior art, we may cite U.S. Pat. No. 3,135,716, which describes the reaction of living diene polymers at the chain end with a polyfunctional organic coupling agent in order to obtain polymers having improved properties; U.S. Pat. No. 3,281,383 which, as coupling or grafting agents of living diene polymers, describes polyepoxides, polyisocyanates, polyimines, polyaldehydes, polyketones, polyanhydrides, polyesters, and polyhalides, including polyhalogenated compositions of silicon; U.S. Pat. No. 3,244,664 which describes tetraalkoxysilanes as coupling and/or starring agents for diene polymers; French Patent 69 24444, which describes, as starring agent, compositions comprising either the --O--C(O)--O-- group or the --O--C(O)--R--C(O)--O-- group and which leads to diene polymers having improved properties of use and improved properties in vulcanized state. In U.S. Pat. No. 4,185,042 there has been described the use of silicon compounds comprising an epoxide group as coupling and/or starring agent of diene polymers for the preparing of coupled and/or starred polymers. The coupling and/or starring agent is present in an amount of 0.05 to 4 moles per mole of living polymer or copolymer, and it is reacted with the living polymer under operating conditions which lead to a coupling and/or starring reaction. The coupled or starred polymers described in that patent have the properties both of an elastomer and of a thermoplastic. No reference is made to filled mixtures, and nothing permits the person skilled in the art to believe that such coupled or starred polymers could be of interest for compositions comprising silica as reinforcing filler. More recently, in the European Polymer Journal, Vol. 17, 1981, pages 213 to 220, A. Crespy and M. J. Abadie have described polystyrene and polyisoprene oligomers functionalized by opening of the epoxy ring by means of 3-glycidoxypropyl-trimethoxysilane (GPTSI) obtained by pouring the living oligomers prepared by anionic catalysis drop-by-drop onto the functionalizing agent, and then deactivating with methanol. It is stated that the yield is improved when GPTSI is placed in solution in tetrahydrofuran (THF). On pages 221 to 227 of this same volume, these authors describe certain polymers functionalized by GPTSI which are obtained by first of all preparing the polymer by anionic polymerization, then subjecting it to metalation by means of naphthalene lithium in excess, and then pouring the living polymer, drop by drop, onto a solution of GPTSI in excess. The functionalization process described by these authors is entirely unusable industrially.
The interest in using silica as reinforcing filler in elastomer compositions has existed for a long time, but the use of silica as reinforcing filler has remained very limited due to the drawbacks mentioned above, and this despite the attempts made to remedy the deficiency of the properties exhibited by such compositions. Thus, it has been proposed to use functionalized diene polymers instead of non-functionalized polymers. By way of example of such a prior art, mention may be made of European Patent Application EP-0 299 074, which describes a silica-filled diene rubber having a base of a diene polymer functionalized by means of a silane having a non-hydrolyzable alkoxy radical. European Patent Application EP-0 447 066 describes a silica-filled composition containing a diene polymer functionalized by means of a halogenated silane compound. Despite the improvement in the properties obtained with the use of such functionalized polymers, the silica compositions described in this prior art have not proved to be usable for forming the treads of tires, since the properties exhibited, although improved, nevertheless remain insufficient with respect to the level required. This interest in silica-reinforced compositions has recently been reawakened with the publication of European Patent Application EP-0 501 227, which discloses a sulfur-vulcanizable rubber composition obtained by thermo-mechanical working of a conjugated diene copolymer and an aromatic vinyl compound prepared by solution polymerization with 30 to 150 parts by weight to 100 parts by weight of elastomer of a special precipitated silica obtained by the process forming the object of European Patent Application EP-0 157 703, which represents an excellent compromise between several contradictory properties and, which for the first time, permits the marketing of tires having a silica-filled tread which actually possesses the required excellent compromise.
It is known to the person skilled in the art that functionalized diene polymers which confer very interesting properties on compositions containing silica as reinforcing filler are obtained with the tetra-alkoxysilanes and in particular tetramethoxysilane (TMSI), used as functionalizing agents. This can be noted, for instance, from U.S. Pat. No. 3,244,664, already cited, which mentions the orthosilicates and describes compositions exhibiting improved properties employing polymers modified with tetraethoxysilane.
The teaching that the tetra-alkoxysilane components are the most interesting potentially for reacting at the chain end with respect to the silica can also be noted from European Patent Application EP-0 299 074, already cited, which in its turn refers to Japanese Patent Application JP-56/104 906, which describes tetraalkoxysilane functionalizing agents and teaches that while these compounds very significantly increase the properties of the polymers, the polymers which are thus functionalized when they are subjected to a steam stripping in order to eliminate the polymerization solvent undergo a macrostructural development which leads to a severe degradation of the potentially interesting properties. Furthermore, this development is very difficult to control.